1. Field of the Invention
The present invention relates to a method of compensating, when a rotating shaft of, for example, a machine tool is positioned at a predetermined angular position, a command value for rotation angle given to the rotating shaft based on an error between, of the rotating shaft, a detected rotation angle detected by a detection device and an actual rotation angle.
2. Description of the Related Art
When a rotating shaft of a machine tool, for example, a lathe's spindle to which a chuck is loaded is brought to and positioned at a predetermined angular position, the spindle is rotationally driven so that a detected rotation angle of the spindle detected by a rotation angle detection device may agree with a command value for rotation angle.
As the aforesaid conventional rotation angle detection device for the spindle, for example, there is one that uses a detection target gear attached to a spindle and having a plurality of teeth formed at a predetermined pitch and a magnetic detection device fixedly disposed at a position facing the teeth of the detection target gear and outputting a voltage signal according to its distance to the teeth (see, for example, Japanese Patent Application Laid-open No. Hei 05-288573).
However, the aforesaid conventional detection device has a problem that between the actual rotation angle of the spindle and the detection value by the detection device, a rotation period error in one rotation of the detection target gear ascribable to the deviation of the center of the detection target gear or the like occurs, and in addition, a tooth-to-tooth period error ascribable to machining accuracy of the teeth of the detection target gear or the like occurs in each tooth-to-tooth period.
As for the compensation of the rotation period error, it is possible to realize the compensation by previously finding detection errors of the detection device at compensation dividing points dividing a rotation period) (360°) of the detection target gear into a plurality of segments and by compensating a command value for rotation angle based on the errors.
On the other hand, as for the compensation of the tooth-to-tooth period error, there arises a problem that currently available rotation angle positioning devices cannot cope with the compensation because the number of decimal places of an interval (deg) between the compensation dividing is points in a tooth-to-tooth period is too large. Further, when the aforesaid method of compensating the rotation period error is adopted as it is for the compensation of the tooth-to-tooth period error, there arises a problem that too large a number of the compensation points hinder the processing.
Thus, the present applicant has found out that a tooth-to-tooth period error pattern presents substantially the same tendency in all the tooth-to-tooth periods and based on this finding, has proposed a method in which a tooth-to-tooth period error pattern in an arbitrary tooth-to-tooth period is employed for all the tooth-to-tooth periods to correct the command value for rotation angle, and thereby the number of the error compensation points can be greatly reduced and only a small memory capacity is required (see Japanese Patent Application Laid-open No. 2011-141247).